1. Field of the Invention
The present invention relates to the field of moving blades for the high pressure turbine of a turbomachine, and more particularly it relates to slots for exhausting cooling air that are situated in the trailing edges of the moving blades of a high pressure turbine.
2. Discussion of the Invention
In conventional manner, a turbomachine has a combustion chamber in which air and fuel are mixed together prior to being burnt therein. The gas that results from this combustion flows downstream inside the combustion chamber and then feeds a high pressure turbine. The high pressure turbine has one or more rows of moving blades spaced apart circumferentially all around the rotor of the turbine. The moving blades of the high pressure turbine are thus subjected to the very high temperatures of the combustion gases. These temperatures reach values well above those which can be withstood without damage by the blades that come into contact with said gas, thereby shortening their lifetime.
In order to solve this problem, it is known to provide these blades with internal cooling circuits seeking to reduce the temperature thereof. By means of such circuits, cooling air, which is generally introduced into a blade via its root, flows along the blade following a path formed by cavities formed in the blade prior to being ejected through slots that open out through the surface of the blade. More precisely, these cooling exhaust slots are generally distributed along the trailing edge of the blade, between its root and its tip, in a manner that is substantially perpendicular to the longitudinal axis of the blade.
It is also known that the blades of a high pressure turbine fitted with cooling circuits are made by molding. The locations of the cooling circuit slots are conventionally reserved by cores placed parallel to one another in the mold prior to casting the metal. In order to make it easier to cast the metal, the cooling air exhaust slot closest to the root of the blade is generally made to have dimensions that are larger than the dimensions of the other slots.
Unfortunately, in practice, it is found that the slot closest to the root of the blade is poorly cooled. Because of the large dimensions of this slot and because of the centrifugal force generated by the blade rotating, air exhausted via this slot tends to be deflected towards the tip of the blade. As a result large temperature gradients arise in the vicinity of the trailing edge which lead to cracking in the vicinity of the slot that is particularly harmful to the lifetime of the blade. These large temperature gradients also tend to propagate by conduction towards the zone where the root of the blade is connected to the platform supporting the blade.
The present invention thus seeks to mitigate such a drawback by proposing a moving blade for a high pressure turbine, the blade presenting a novel shape for the cooling air exhaust slot closest to the root of the blade, which slot does not lead to cracking. The invention also seeks to avoid degrading the general mechanical strength of the blade which is a part that is subjected to very high levels of mechanical stress. Finally, the invention seeks to provide a high pressure turbine for a turbomachine fitted with such moving blades.
To this end, the invention provides a moving blade for a high pressure turbine of a turbomachine, the blade having at least one cooling circuit comprising at least one cavity extending radially between a tip and a root of the blade, at least one air admission opening at one of the radial ends of the cavity(ies) to feed the cooling circuit(s) with cooling air, and a plurality of slots opening out from the cavity(ies) and into the trailing edge of the blade, the slots being arranged along the trailing edge between the root and the tip of the blade in a manner that is substantially perpendicular to a longitudinal axis of the blade, wherein at least the slot closest to the root of the blade presents an inclination towards the tip of the blade lying in the range 10xc2x0 to 30xc2x0 relative to an axis of rotation of the blade.
As a result, the cooling air exhausted through the slot closest to the root of the blade is guided over the entire surface of the slot so as to avoid cracking appearing therein. This particular shape for the slot makes it possible to reduce the local temperature around said slot by about 5%. In addition, the ability of the blade to withstand the various mechanical stresses to which it is subjected is not degraded by this shape of slot.
Advantageously, the inclination of the slot closest to the root of the blade is about 20xc2x0.
In order to lower the temperature of a connection zone between the root of the blade and a platform defining the flow stream of combustion gases through the high pressure turbine, the upstream end of the slot closest to the root of the blade is essentially formed in said connection zone.